This invention relates generally to apparatus for tightening fastener systems and, more particularly, to apparatus for tightening fastener systems to the yield point or some similarly significant point characterized by a significant change in slope of the torque-time curve plotted for that particular fastener being tightened and corresponding to a predetermined axial load on the fastener.
In the design of structural joints secured by mechanical fastener systems, it is usual to provide for the fasteners to exert a predetermined clamping force or laod on the structural members in order to insure the integrity of the joint. When a joint is assembled, it is therefore desirable that the fasteners be tightened to exert a predetermined axial load on the associated structural members. However, many prior art tightening techniques for tightening threaded fasteners, such as nuts and bolts, to exert a predetermined load on associated structural members are not entirely satisfactory. For example, the most accurate tightening technique involves measuring the axial strain or stretch of the bolt while it is being tightened and relating the stretch to the stress or axial load acting on the bolt through previously calculated stress-strain relationships. While this technique is most accurate, practical applications do not usually permit measurement of the stretch of the bolt and, in those instances where the stretch can be measured, it is a time consuming and relatively expensive technique. Accordingly, this technique is used in relatively few applications outside of laboratory testing.
Another known tightening technique that is comonly used in assembling the majority of joints involves the use of torque control tools, which indicate when the torque applied to the fastener equals or exceeds a predetermined value, and stopping tightening of the fastener in response thereto. Torque measurement is relatively easy and since torque is related to the axial force induced in the fastener assembly, and exerted on the structural members, the predetermined torque can be selected to theoretically correspond to the predetermined clamp load specified for the joint. However, when tightening threaded fasteners in an assembly line operation, wide variations in the actual torque-load relationship are experienced. These variations are caused by a variety of factors including allowable tolerence variations in the dimensions and strength of the fasteners and structural members, and lubrication or absence thereof on the mating surfaces of the fasteners and/or the structural members. All of these factors can cause large variations in the coefficient of friction between the mating surfaces of the fastener and the joint. In actual practice, variations of up to plus or minus 30% in the axial load on the bolts used for a particular application can be experienced at the same torque level. Accordingly, the torque control technique is not very accurate.
In an effort to overcome the problems associated with the prior art tightening systems, other tightening systems have been developed that include the use of tools measuring both the torque and angular displacement, or rotation, of a fastener during the tightening cycle. These tightening systems include control systems operative in response to the torque and angle measurements to determine when the slope of a torque-rotation curve for the fastener indicates that the yield point of the fastener has been reached and to then stop tightening the fastener. Neither the techniques nor the tools disclosed in the prior art patents are generally satisfactory for accomplishing the desired objective because they are not adaptive systems. That is, in each of the disclosed techniques it is necessary to know in advance either the torque-rotation relationship for the particular fastener being tightened or the torque gradient at the yield point. The torque-rotation relationship varies over a wide range for the same reasons that the torque-load relationships vary and, accordingly the techniques and tools disclosed in the prior art patents can be utilized only where the characteristics of the joint assembly are known in advance and average relationships must be predetermined and utilized in the operation of the tools. Thus the versatility and accuracy of the techniques and tools disclosed in the prior art patents are not fully satisfactory.
Another tightening system is disclosed in U.S. Pat. No. 3,782,419 "Apparatus For And Method Of Determining Rotational or Linear Stiffness" by John T. Boys. In this system, signals of both the torque applied and angular rotation of a fastener are provided during the tightening cycle in order to develop a signal indicative of the gradient of a curve which could be plotted for the two parameters. An instantaneous gradient signal is compared with a stored gradient signal and the tightening system is shut off in response thereto. The present invention is an improvement over the above-described system in that the present system does not require angular rotation signals obtained from the wrench means, as is disclosed in the above-identified application. The present control system utilizes a relatively inexpensive oscillator, which is not connected to the wrench means, for providing a second input parameter-time. Time signals are used, along with measured torque signals, in the gradient calculating portion of the control system to develop a gradient signal for controlling the operation of the present tightening system.